This invention relates, generally, to calibration techniques for velocity controllers, and more specifically, to such techniques as applied to electric motors used in printers.
Computer printers used a variety of motors to actuate pumps, move components, and perform many other functions. In one type of configuration, a Pulse Width Modulated (PWM) signal is supplied to a DC motor. The DC motor may be used, for example, to power a mechanism that primes and purges the print head. In such an application the mechanism will have a well-defined travel path and pre-set hard stops.
With these conditions, it is important that the PWM signal is sufficient w so that the motor moves the mechanism to its various stops. However, if the PWM signal is too strong the motor can move the mechanism too quickly. These movements can cause ink to be thrown off of the mechanism or can cause damage to the mechanism or other components.
To control the speed of motors one can employ a proportional integral controller which uses a feedback system based on the motor speed. However, the sensors used to detect the speed can be expensive and can add to the sophistication of the overall system. In low-cost applications these sensors can also create a degree of velocity control and position control which is beyond the requirements of the application. In the printer purge application one is concerned about the motor having velocity in a proper range. However, precise velocity control and precise position control are not as great a concern as they might be in other applications.
It is possible to have sufficient control over a motor for printer purge applications without using expensive sensors. With the addition of an adequate technique to overcome measurement noise, motor variation, and load variations, a feedback system based on the current in the motor can be satisfactory. However, no known system based on current sensing only can achieve the necessary results without the use of a relatively expensive motor.
What is needed in the printer purge and other similar applications is a calibration technique which enables a velocity controller to overcome the limitations of inexpensive motor designs, particularly 3-pole DC motors, so as to permit sufficient velocity control to keep the motor within a range that prevents undesirable consequences but which does not require expensive sensors.
It is an object of the present invention to provide a calibration technique for electric motor velocity controllers which allows for relatively precise control over inexpensive motor designs.
It is another object of the present invention to provide a calibration, technique for velocity controllers using current sensing only.
It is another object of the present invention to provide a calibration technique for velocity controllers which do not require the use of optical or other sophisticated sensors.
It is another object of the present invention to provide a calibration technique that will enable a velocity controller to operate properly in the presence of measurement noise, motor variations, and load variations.
A calibration technique for electric motor velocity controllers is provided. The method gives velocity controllers that are limited to current sensing only a level of accuracy sufficient for many applications, even when utilizing inexpensive motors.
The invention is based on the concept that variations in the motor parameters and the current sensing circuitry cause variations in the apparent resistance of the motor. By identifying the true value of the apparent resistance, the mapping function can be adjusted for optimum performance. This is accomplished through the application of a specific cycle of pulse-width modulated voltages (PWM), as is explained more fully below.
An advantage of the invention is that it eliminates the need for optical or other more complicated velocity sensors.
A further advantage of the invention is that it enables a velocity controller using current sensing only to sufficiently direct relatively inexpensive motors.
A further advantage of the invention is that it enables a velocity controller to operate properly in the presence of measurement noise, motor variations, and load variations.
These and other objects, advantages, and features of this invention will be apparent from the following description.